The present invention relates to a discrete multitone (DMT) system, and more particularly, to an adaptive bit swapping method and device for a DMT system, which adjust the number of bits and power assigned to each sub-channel according to channel characteristics varied during a data transmission.
A multicarrier is generally used in a DMT system to efficiently use a channel for transmitting data. Basically in multicarrier modulation, several carrier-modulated waveforms are overlapped to represent an input bit stream. A multicarrier transmission signal is the composite of M independent sub-signals or sub-channels, each having the same bandwidth of 4.3125 KHz and the respective main frequencies of f.sub.i (i=1, 2, 3,. . .M). These sub-signals are Quadrature Amplitude Modulation (QAM) signals. When data is transmitted at a high speed via an inferior transmission path such as a copper line, the DMT system enables the data to be transmitted at 6 Mbps or above, thus offering a good service. In this DMT system using several carriers, the number of bits and power of each channel depending on its signal-to-noise ratio (SNR) are assigned to each sub-channel in the initialization of the system.
Changing the number of bits and power assigned to each sub-channel according to its SNR, which is varied without an interruption in a data stream in a data transmission mode, is referred to as bit swapping. The bit swapping is used in an Asymmetric Digital Subscriber Line (ADSL) service employing the DMT system to reduce an error probability of transmission data.
Channel characteristics gradually vary with time in most systems, and frequency response characteristics of an ADSL loop gradually vary with temperature. Therefore, a channel model determined in the initialization of a system should be changed according to the frequency response characteristics.
A conventional method for allocating bits to a sub-channel will be described as follows.
A transmitter terminal as well as a receiver terminal adaptively can operate according to the essential concept of a bit allocating method which is proposed in a dissertation submitted to the department of electrical engineering and the committee on graduate studies of STANFORD University in partial fulfillment of the requirements for the degree of doctor of philosophy, May. 1993, entitled "BANDWIDTH OPTIMIZED DIGITAL TRANSMISSION TECHNIQUES FOR SPECTRALLY SHAPED CHANNELS WITH IMPULSE NOISE", by Ronald R, Hunt and P.S. Chow. Details of the bit allocating method are as follows:
1. the steady state mean square errors(MSE)'s of all used sub-channels are monitored, where these error values are differences between inputs and outputs of a slicer; PA1 2. it is continuously determined whether the difference between a maximum error value and a minimum error value is a predetermined threshold value (generally 3 dB) or above, and if the difference is the threshold value or above, the procedure goes to the subsequent step; PA1 3. the bit number of a value in a bit allocation table for a sub-channel having the maximum error value is decreased by 1, while the bit number of a value in a bit allocation table for a sub-channel having the minimum error value is increased by 1; PA1 4. the minimum error is doubled, while the maximum error is halved; PA1 5. the slicer settings for two sub-channels whose bit values are changed are adjusted; and PA1 6. the bit swapping information is sent back to a transmission part.
The initial number of bits allocated to a sub-channel is determined according to an SNR measured during an initialization in an ADSL DMT system. However, the above bit allocation method exhibits the drawback that a wrong bit swapping may be performed, since an MSE value may be increased due to an error such as a burst error when data is examined in a reception part, and a frequency-domain equalizer (FEQ) error can affect MSE in a steady state.